Compact distributed bragg reflectors

ABSTRACT

Ultra compact DBRs, VCSELs incorporating the DBRs and methods for making the DBRs are provided. The DBRs are composed of a vertical reflector stack comprising a plurality of adjacent layer pairs, wherein each layer pair includes a layer of single-crystalline Group IV semiconductor and an adjacent layer of silicon dioxide.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under FA9550-09-1-0482awarded by the USAF/AFOSR. The government has certain rights in theinvention.

BACKGROUND

Distributed Bragg Reflectors (DBRs) are used is photodetectors, lasers,solar cells, and other optical sensors. DBRs include multiple layers ofalternating materials that have different refractive indices, thethickness of which typically varies from several to several tens ofmicrometers, depending on the materials. There are several parametersthat determine the performance of DBRs, such as refractive indexcontrast, the roughness of the top surface, and the condition of theinterfaces between materials. Epitaxial growth makes it possible tofabricate group III-V semiconductor-based photonic devices, includingDBRs, on group III-V material substrates. However, these DBRs typicallyrequire many (e.g., >20) layers of material and need to be quite thick(e.g. >10 μm thick) to achieve greater than 90% reflectivity. DBRsconstructed from silicon and silicon oxide layers, on the other hand,have a high refractive index contrast, so that they provide highreflection with a more compact structure, compared to DBRs constructedfrom group III-V materials. DBRs based on pairs of silicon and silicondioxide (Si/SiO₂ DBRs) have been fabricated using chemical vapordeposition (CVD) to deposit polycrystalline (poly-) Si as the siliconlayers. (J. C. Bean, J. Qi, C. L. Schow, R. Li, H. Nie, J. Schaub, andJ. C. Campbell, “High-speed polysilicon resonant-cavity photodiode withSiO₂/Si Bragg reflectors,” Photonics Technology Letters, IEEE 9, 806-808(1997).) However, the roughness of the layer surfaces in such DBRsincreases with increasing number of layers. (S. Akiyama, F. J. Grawert,J. Liu, K. Wada, G. K. Celler, L. C. Kimerling, and F. X. Kaertner,“Fabrication of highly reflecting epitaxy-ready Si—SiO₂ Braggreflectors,” Photonics Technology Letters, IEEE 17, 1456-1458 (2005).)In addition, structural imperfections in the CVD-grown materials, suchas local variations in the refractive index, grain boundaries, stackingfaults, dislocations, crystallographic twinning, mosaic misorientation,and localized defects, degraded the overall DBR performance byincreasing undesirable scattering. (G. Harbeke, “Optical properties ofpolycrystalline silicon films,” in Polycrystalline Semiconductors(Springer, 1985), pp. 156-169.) To solve the issues related to interfaceroughness and post-epitaxial growth, Si/SiO₂ DBRs were made usingmultiple oxygen implantations into a single-crystalline Si substrate atdifferent depths. (Y. Ishikawa, N. Shibata, and S. Fukatsu,“Epitaxy-ready Si/SiO₂ Bragg reflectors by multipleseparation-by-implanted-oxygen,” Appl. Phys. Lett. 69, 3881-3883(1996).) However, limitations on the choice of Si and SiO₂ layerthicknesses prevent this method from achieving ideal design parameterssince the interface morphology is dependent on the thickness of bothlayers. A DBR having a single-crystalline top Si layer andpolycrystalline lower Si layers has been fabricated using CVD on top ofa silicon-on-insulator (SOI) wafer, followed by etching back the entireSi substrate layer and buried oxide layer to expose the top Si layer.(S. Akiyama, F. J. Grawert, J. Liu, K. Wada, G. K. Celler, L. C.Kimerling, and F. X. Kaertner, “Fabrication of highly reflectingepitaxy-ready Si—SiO₂ Bragg reflectors,” Photonics Technology Letters,IEEE 17, 1456-1458 (2005).) This DBR had 7 pairs of Si/SiO₂. To reducethe total DBR thickness, DBRs made using a Smart-Cut process have beenproposed to insert a single-crystalline Si in each layer pair. (M. K.Emsley, O. Dosunmu, and M. S. Unlu, “Silicon substrates with burieddistributed Bragg reflectors for resonant cavity-enhancedoptoelectronics,” Selected Topics in Quantum Electronics, IEEE Journalof 8, 948-955 (2002).) In addition, DBRs fabricated with transferredsingle-crystalline silicon layers and amorphous Spin-on-Glass (SOG)silicon oxide (SiO_(x)) layers have been disclosed. (W. Peng, M. M.Roberts, E. P. Nordberg, F. S. Flack, P. E. Colavita, R. J. Hamers, D.E. Savage, M. G. Lagally, and M. A. Eriksson, “Single-crystalsilicon/silicon dioxide multilayer heterostructures based onnanomembrane transfer,” Appl. Phys. Lett. 90, -(2007).) However, it isdifficult to precisely control the thickness of an SOG oxide layer andthe non-uniform SOG oxide surface left voids at the Si/SiO_(x) interfaceafter the transfer of the silicon layer. Such voids may affect the DBRperformance and the bonding strength between the two layers.

SUMMARY

Ultra compact DBRs, vertical cavity surface emitting lasers (VCSELs)incorporating the DBRs and methods for making the DBRs are provided.Also provided are DBR arrays and VCSEL arrays.

One embodiment of a DBR comprises: a reflector stack comprising at leasttwo adjacent layer pairs, each layer pair comprising: a layer of asingle-crystalline Group IV semiconductor, such as silicon or germanium;and an adjacent layer of silicon dioxide. The thickness of the reflectorstack may be less than 1.2 μm and in some embodiments is no greater than1 μm. In some embodiments of the stacks, the rms roughness at theinterfaces between the layers of single-crystalline Group IVsemiconductor and the layers of silicon dioxide is no greater than 0.5nm. In an embodiment of a DBR array, a plurality of the DBRs is providedon a substrate, wherein different reflectors in the array are configuredto reflect light over different wavelength ranges.

One embodiment of a VCSEL comprises a lower DBR comprising a reflectorstack comprising at least two adjacent layer pairs, each layer paircomprising: a layer of a single-crystalline Group IV semiconductor; andan adjacent layer of silicon dioxide; wherein the thickness of thereflector stack is no greater than 1 μm and the rms at the interfacesbetween the layers of single-crystalline Group IV semiconductor and thelayers of silicon dioxide is no greater than 0.5 nm. The VCSEL furthercomprises an upper DBR comprising a reflector stack comprising least twoadjacent layer pairs, each layer pair comprising: a layer of asingle-crystalline Group IV semiconductor; and an adjacent layer ofsilicon dioxide; wherein the thickness of the reflector stack is nogreater than 1 μm and the rms roughness at the interfaces between thelayers of single-crystalline Group IV semiconductor and the layers ofsilicon dioxide is no greater than 0.5 nm. The VCSEL further comprises alight-emitting active layer disposed between the lower DBR and the upperDBR. In an embodiment of a VCSEL array, a plurality of the VCSELSs isprovided on a substrate, wherein different VCSELs in the array areconfigured to emit light at different wavelengths when a voltage isapplied across the VCSEL.

One embodiment of a method of making a DBR comprises the steps of:growing a first layer of silicon dioxide on a first layer ofsingle-crystalline silicon via thermal oxidation of thesingle-crystalline silicon; transferring a second layer ofsingle-crystalline silicon onto the first layer of silicon dioxide;growing a second layer of silicon dioxide on the second layer ofsingle-crystalline silicon via thermal oxidation of thesingle-crystalline silicon; and transferring a third layer ofsingle-crystalline silicon onto the second layer of silicon dioxide.

Another embodiment of a method of making a DBR comprises the steps of:depositing a first layer of silicon dioxide on a first layer ofsingle-crystalline germanium; transferring a second layer ofsingle-crystalline germanium onto the first layer of silicon dioxide;depositing a second layer of silicon dioxide on the second layer ofsingle-crystalline germanium; and transferring a third layer ofsingle-crystalline germanium onto the second layer of silicon dioxide.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings, wherein like numeralsdenote like elements.

FIG. 1 is a schematic diagram of a transfer and bonding based method offabricating a DBR composed of 2.5 Si/SiO₂ layer pairs.

FIG. 2 is a schematic diagram of a vertical cavity surface emittinglaser that includes upper and lower DBR composed of Si/SiO₂ layer pairs.

FIG. 3 is a schematic diagram of a DBR array.

FIG. 4 is a scanning electron microscope (SEM) image of a cross-sectionof a DBR composed of Si/SiO₂ layer pairs.

FIG. 5 is an optical microscope image of a DBR composed of Si/SiO₂ layerpairs.

FIG. 6A shows the measured (upper line) and FDTD simulated (lower line)reflection spectra for a single-pair Si/SiO₂ DBR with a siliconsubstrate in the range from 1500 to 1600 nm. FIG. 6B shows thereflection spectra in the range from 800 nm to 2400 nm.

FIG. 7A shows the measured (upper line) and FDTD simulated (lower line)reflection spectra for a two-pair Si/SiO₂ DBR with a silicon substratein the range from 1500 to 1600 nm. FIG. 7B shows the reflection spectrain the range from 800 nm to 2400 nm.

FIG. 8A shows the measured (upper line) and FDTD simulated (lower line)reflection spectra for a three-pair Si/SiO₂ DBR with a silicon substratein the range from 1500 to 1600 nm. FIG. 8B shows the reflection spectrain the range from 800 nm to 2400 nm.

FIG. 9A shows the measured (lower line) and FDTD simulated (upper line)reflection spectra for a 1.5-pair Si/SiO₂ DBR with a quartz substrate inthe range from 1500 to 1600 nm. FIG. 9B shows the reflection spectra inthe range from 800 nm to 2400 nm.

FIG. 10A shows the measured (upper line) and FDTD simulated (lower line)reflection spectra for a 2.5-pair Si/SiO₂ DBR with a quartz substrate inthe range from 1500 to 1600 nm. FIG. 10B shows the reflection spectra inthe range from 800 nm to 2400 nm.

FIG. 11A shows the measured (lower line) and FDTD simulated (upper line)reflection spectra for a 5.5-pair poly-Si/SiO₂ DBR with a quartzsubstrate in the range from 1500 to 1600 nm as a comparison. FIG. 11Bshows the reflection spectra in the range from 800 nm to 2000 nm.

DETAILED DESCRIPTION

Ultra compact DBRs, VCSELs incorporating the DBRs and methods for makingthe DBRs are provided. The DBRs are composed of a vertical reflectorstack comprising a plurality of adjacent layer pairs, wherein each layerpair includes a layer of single-crystalline Group IV semiconductor, suchas silicon or germanium, and an adjacent layer of silicon dioxide (a“Si/SiO₂” pair or “Ge/SiO₂” pair). The single-crystalline silicon layersare incorporated into the stack using a thin film transfer and bondingprocess, while the SiO₂ layers can be thermally grown from the siliconor deposited. These fabrication schemes make it possible to obtain highquality silicon dioxide and precisely controlled and uniform layerthicknesses, which minimizes scattering loss at the interfaces withinthe reflector stack. As a result, the DBRs can be very thin, with fewlayer pairs, yet still provide extremely high reflectance. In addition,the transfer and bonding process makes it possible to dispose the DBRson a wide variety of substrates.

A method of making a DBR comprising Si/SiO₂ pairs is illustratedschematically in FIG. 1. As shown in panel (a), the method begins withan SOI substrate comprising a handle substrate 102, a buried siliconoxide layer 104 and thin layer of single-crystalline silicon 106. SOIsare available commercially. Buried oxide layer 104 is selectivelyremoved from the structure using, for example, a selective chemicaletchant. As a result, silicon layer 106 settles onto underlying handlesubstrate 102, as shown in panel (b). If necessary, the layer ofsingle-crystalline silicon 106 can be thinned using, for example, apolish or etch before it is release from the SOI structure. Aftersilicon layer 106 is released, a host material 108, such as a rubberstamp, is pressed onto the upper surface of silicon layer 106. Siliconlayer 106 adheres to host material 108 and is lifted away from handlesubstrate 102 (panel (c)). In a subsequent step (panel (d)) releasedsingle-crystalline silicon layer 106 is brought into contact with, andtransferred onto, a support substrate 110. Support substrate 110 may becomposed of a material with a high refractive index, such as quartz orsilicon. Next, as shown in panel (e), a thin layer of silicon dioxide112 is grown on transferred single-crystalline silicon layer 106. Asecond layer pair is then formed on silicon dioxide layer 112 bytransferring a second single-crystalline silicon layer 114 onto silicondioxide layer 112 (panel (f)) and then growing a second silicon dioxidelayer 116 on silicon layer 114 via thermal oxidation (panel (g)). In theembodiment shown in FIG. 1, a third layer of single-crystalline silicon118 is the transferred onto the second layer of silicon dioxide 116 toprovide a reflector stack having 2.5 layer pairs (panel (h)). However,in other embodiments the silicon layer transfer-and-thermal oxidationsteps can be repeated a number of times to provide a reflector stackwith 3, 3.5, 4 or even more layer pairs. Although the silicon thin filmrelease-and-transfer process illustrated in FIG. 1 uses a dry transfer,a wet transfer could also be used, followed by drying.

Because the silicon layer in each layer pair is a layer ofsingle-crystalline silicon, it is distinguishable from silicon layerscomposed of either polycrystalline or amorphous silicon. In addition,because the silicon dioxide layer is formed via thermal growth, it is ahigh quality silicon dioxide and is, therefore, distinguishable fromspin-on-glass type silicon oxides, which comprise mixtures ofnon-stoichiometric silicon oxides, commonly denoted as SiO_(x).

A DBR comprising Ge/SiO₂ pairs can be fabricated using the samerelease-and-transfer steps shown in panels (a), (b), (c), (d), (f) and(h) of FIG. 1 to make the germanium layers. The SiO₂ layers can then bedeposited on the germanium layers using, for example, plasma-enhancedchemical vapor deposition (PECVD), sputtering, atomic layer deposition(ALD), or evaporation.

The DBRs include a reflector stack having at least two layer pairs andhave a thickness of less than 1 μm. This includes embodiments having areflector stack thickness of no greater than 0.9 μm, further includesembodiments having a reflector stack thickness of no greater than 0.8μm. In some embodiments, reflector stacks having such small thicknesseshave 2.5 layer pairs, while in other embodiments the reflector stackshave 3 layer pairs.

For DBRs comprising Si/SiO₂ pairs, the thickness of each layer withinthe stack and the relative thicknesses of the single-crystalline siliconlayer and the SiO₂ layers can be carefully controlled, such that theSiO₂ layers are thicker than, for example, at least twice as thick as,the single-crystalline silicon layers. By way of illustration, in someembodiments, the SiO₂ layer in each layer pair has a thickness of 450 orless and the single-crystalline silicon layer in each layer pair has athickness of 200 nm or less. This includes embodiments in which the SiO₂layer in each layer pair has a thickness of no greater than 300 nm andthe single-crystalline silicon layer in each layer pair has a thicknessof no greater than 150 nm. For example, one embodiment of a DBR has twofull Si/SiO₂ layer pairs and an additional layer of single-crystallinesilicon (a half layer pair) to provide a DBR with 2.5 layer pairs. Ifthe Si layers are 110 nm thick and the SiO₂ layers are 270 nm thick, theDBR will exhibit high reflectivity at 1.55 μm. If the Si layers are 175nm thick and the SiO₂ layers are 435 nm thick, the DBR will exhibit highreflectivity at 0.98 μm.

Because germanium has a higher refractive index than silicon, the DBRscomprising Ge/SiO₂ pairs can achieve similar reflectivities and slightlywider bandwidths using thinner layer pairs (e.g, ˜30% thinner). By wayof illustration, in some embodiments, the SiO₂ layer in each layer pairhas a thickness of 100 nm or less and the single-crystalline germaniumlayer in each layer pair has a thickness of 120 nm or less. For example,one embodiment of a DBR has two full Ge/SiO₂ layer pairs and anadditional layer of single-crystalline germanium (a half layer pair) toprovide a DBR with 2.5 layer pairs. If the Ge layers are 110 nm thickand the SiO₂ layers are 90 nm thick, the DBR will exhibit highreflectivity at 1.55 μm. If the Ge layers are 175 nm thick and the SiO₂layers are 145 nm thick, the DBR will exhibit high reflectivity at 0.98μm.

The stacks are characterized by extremely smooth interfaces between theGroup IV semiconductor and silicon dioxide layers. Is some instances,these interfaces are as smooth as those provided by epitaxial growth.For example, the Si/SiO₂ interfaces or the Ge/SiO₂ interfaces may havean rms roughness of no greater than about 0.5 nm. This includesembodiments in which the Si/SiO₂ interfaces or the Ge/SiO₂ interfaceshave an rms roughness of no greater than about 0.3 nm. The rms roughnessof interface can be determined from an AFM image over the area of theinterface.

Because the DBRs are not formed by epitaxial growth, which requires alattice match (or near lattice match) with a growth substrate, thereflector stacks can be formed on a wide variety of substrates. Suitablesubstrates include electrically conductive (e.g., metal) substrates,dielectric substrates and semiconducting substrates. In someembodiments, the reflector stacks are disposed on a silicon substrate orother group IV material (e.g., germanium or diamond) substrate. However,they can also be disposed on, for example, a substrate of a group III-Vsemiconductor or a group II-VI semiconductor. Such substrates may bevery thin and mechanically flexible.

The DBRs are characterized by high reflectance for wavelengths in therange from about 900 to about 2200 nm—particularly in the range fromabout 1300 to about 1700 nm and more particularly in the range fromabout 1400 to about 1670. The high reflectance can be measured aspercent reflectivity on a given substrate, such as silicon or quartz, asillustrated in the Example below or, is the DBRs are incorporated into alaser, such as a VCSEL, the percent reflectivity refers to thereflectivity of the light emitted by the light-emitting active region ofthe laser. Some embodiments of the DBRs provide a reflectivity of atleast 99.5% at a wavelength in these ranges, on a silicon or quartzsubstrate. This includes DBRs that provide a reflectivity of at least99.6% at a wavelength of 1550 nm on a quartz substrate and DBRs thatprovide a reflectivity of at least 99.6% at a wavelength of 980 nm on aquartz substrate.

The DBRs can be used as passive, stand-alone mirrors that are notincorporated into an electronic or photonic device. Such stand-alonemirrors can be used, for example, in weapons defense applications todeflect incoming laser beams. However, the DBRs can also be integratedinto electronic or photonic devices. For example one or more DBRs can beused as the lower and/or upper reflectors in a VCSEL, such that thelight-emitting active region of the VCSEL is disposed between the tworeflectors. A schematic diagram of one embodiment of a VCSEL comprisinga multiple quantum well (MQW) pin diode structure is shown is FIG. 2.The pin diode structure of the VCSEL comprises a hole injection layercomprising a single-crystalline p-type doped semiconductor material 202;an electron injection layer comprising a single-crystalline n-type dopedsemiconductor material 204; and an intrinsic light-emitting activeregion 206 disposed between the hole injection layer and the electroninjection layer.

Intrinsic active region 206 includes a MQW structure 205 comprisingalternating barrier and quantum well layers, which are made of differentsemiconductor materials. In the MQW structures, charge carriers areconfined via quantum confinement in thin layers of one semiconductor“well” material sandwiched between layers of other semiconductor“barrier” material. The active region may further comprise a lowerspacer layer 207 and an upper spacer layer 208, between which the MQWstructure is disposed. The spacer layers are used to increase thethickness of the intrinsic active region and, because they form part ofthe intrinsic active region, they are comprised of undopedsingle-crystalline semiconductor materials. Lower DBR 210 is disposedbeneath active region 206 and upper DBR 212 is disposed over activeregion 206. A dielectric material may be provided around the MQW pindiode structure to provide electrical insulation. Finally, a p-type ringcontact 216 and a n-type ring contact 218 are formed on hole injectionlayer 202 and electron injection layer 204, respectively. In someembodiments, the innermost Group IV semiconductor layers 220 of thereflector stack in one or both DBRs is doped (n-type or p-type) toprovide ohmic contacts. In these embodiments, charge injection layers202 and 204 can be omitted and the ring contacts can be in electricalcontact with the doped layers of the DBRs.

The intrinsic active region and the n-type and p-type doped chargeinjection layers may be fabricated on a suitable growth substrate usingknown methods, such as molecular beam epitaxy (MBE). The DBRs areincorporated into the VCSEL using a transfer and bonding process. Forexample, the reflector stacks can be pre-fabricated using the processshown in FIG. 1 and then transferred from their support substrate 110onto a MQW pin diode structure. Alternatively, the MQW pin diodestructure can be used as support substrate 110, such that the Si/SiO₂pairs or Ge/SiO₂ pairs of the reflector stacks are formed directly onthat structure.

Because the DBRs are incorporated into the VCSEL via a transfer andbonding process, the interfaces formed between the reflector stacks andthe hole and electron injection layers do not have an epitaxialstructure. As used herein the term “epitaxial structure” refers to astructure in which the crystallographic orientation of an overlyinglayer is determined by (matches) that of its underlying layer, such thatthe two layers have the same crystallographic orientation, at least inthe area of their interface. Such epitaxial structures may includestrains and stresses at the interface, induced by a lattice mismatchbetween the two materials and may even include misfit dislocations. Incontrast to such epitaxial interfaces, non-epitaxial interfaces in thepresent structures have crystallographic orientations that areindependent from (e.g., different from) those of their neighboringlayers. As a result, the semiconductor materials of the MQW pin diodestructure need not be lattice matched to the silicon or germanium of theDBRs. For example, semiconductor materials of the active region and/orthe hole and/or electron injection layers can be independently selectedfrom a broad range of semiconductors including: (a) group IVsemiconductors; (b) group III-V semiconductors; and (c) group II-VIsemiconductors. The group IV semiconductors include elementalsemiconductors (e.g., Si, Ge and C, including diamond), as well as alloyand compound semiconductors (e.g., SiGe:C, SiGe, SiGeSn and SiC). Thegroup III-V and group II-VI semiconductors include binary, ternary andhigher compound semiconductors. Examples of group III-V semiconductorsinclude GaAs, AlGaAs, InGaAs, AlAs, InAlAs, InP, GaInP, GaP, GaN, InGaN,InAIN, MN and AlGaN. Examples of group II-VI semiconductors includeoxides, such as ZnO. For example, a VCSEL configured to emit infraredradiation in the wavelength range from about 1400 to 1670 nm couldemploy an active region having an MQW structure comprising alternatinglayers of single-crystalline InGaAs quantum well layers andsingle-crystalline InP barrier layers.

The transfer and bonding-based fabrication scheme makes it possible tofabricate a DBR array comprising a plurality (for example at least ten,at least 100, at least 1000) of DBRs on the same substrate, whereindifferent DBRs in the array are configured to reflect radiation havingdifferent wavelength ranges. For example the DBR array may comprise oneor more DBR comprising Si/SiO₂ pairs and one or more DBR comprisingGe/SiO₂ pairs. Additionally, (or alternatively) the DBR array maycomprises one or more DBRs comprising Si/SiO₂ pairs, wherein one or moreof the DBRs are configured to reflect radiation over a first wavelengthrange and one or more of the DBRs are configured to reflect radiationover a second wavelength range that differs from said first wavelengthrange. Similarly, the DBR array may comprises one or more DBRscomprising Ge/SiO₂ pairs, wherein one or more of the DBRs are configuredto reflect radiation over a first wavelength range and one or more ofthe DBRs are configured to reflect radiation over a second wavelengthrange that differs from said first wavelength range. An embodiment of aDBR array of this type is illustrated in FIG. 3.

The DBR array includes a substrate 302 and a plurality of DBRs on thesurface of substrate 302. The first DBR 304 comprises 2.5 Si/SiO₂ layerpairs and the second DBR 306 also comprises 2.5 Si/SiO₂ layer pairs. Thefirst and second layer pairs are different thicknesses because they areconfigured to reflect different wavelength ranges. The third DBR 308comprises 2.5 Ge/SiO₂ layer pairs and the fourth DBR 310 also comprises2.5 Ge/SiO₂ layer pairs. The third and fourth layer pairs are differentthicknesses because they are configured to reflect different wavelengthranges.

The transfer and bonding-based fabrication scheme also makes it possibleto fabricate a VCSEL array comprising a plurality (for example at leastten, at least 100, at least 1000) of VCSELs on the same substrate,wherein different VCSELs in the array are configured to emit radiationhaving different wavelengths. For example the VCSEL array may compriseone or more VCSELs comprising Si/SiO₂ pairs in their DBRs and one ormore VCSELs comprising Ge/SiO₂ pairs in their DBRs. Additionally, (oralternatively) the VCSEL array may comprises one or more VCSELscomprising DBRs having Si/SiO₂ pairs, wherein one or more of the VCSELsare configured to emit radiation at a first wavelength and one or moreof the VCSELs are configured to emit radiation at a second wavelengththat differs from said first wavelength. Similarly, the VCSEL array maycomprises one or more VCSELs comprising DBRs having Ge/SiO₂ pairs,wherein one or more of the VCSELs are configured to emit radiation at afirst wavelength and one or more of the VCSELs are configured to emitradiation at a second wavelength that differs from said firstwavelength.

EXAMPLES

This example illustrates methods of making high reflectivity, ultra-thinDBRs using a thin layer transfer and bonding process in combination withthermal oxide growth.

Simulations. In order to obtain layer parameters for the best DBRreflection, simulations were carried out with MEEP software (afinite-difference time-domain (FDTD) simulation software packagedeveloped and distributed by MIT). Si/SiO₂ layer thicknesses of 110nm/270 nm were extracted and a reflectance of greater than 99% waspredicted for a 2.5-layer pair DBR on a quartz substrate and a 3-layerpair DBR on a Si substrate.

Materials and Methods. The Si/SiO₂ DBRs were fabricated according to theprocess illustrated in FIG. 1. Fabrication started by thinning down thetop single-crystalline Si layer of SOI wafer (SOITEC, 340 nm/2000 nm,Si/buried oxide) from 340 nm down to 234 nm using thermal oxidation andwet etching with a hydrofluoride solution (HF, 49%). The SOI sample wasimmersed in HF to undercut the buried oxide layer and, eventually,release the thinned single-crystalline Si layer. The released Si layerhad an area of about 1 mm² The layer of single-crystalline Si settledonto the underlying handle wafer of the SOI and was rinsed withdeionized (DI) water. Then the thin layers of single-crystalline Si weretransferred onto quartz or silicon support substrates using apolydimethylsiloxane (PDMS) stamp as an intermediate host substrate. Forthe DBRs on silicon substrates, a 270 nm thermal oxide was grown on thesubstrate prior to the transfer of the single-crystalline silicon layer.A detailed description of the transfer procedure can be found in K.Zhang, J.-H. Seo, W. Zhou, and Z. Ma, “Fast flexible electronics usingtransferrable silicon nanomembranes,” J. Phys. D: Appl. Phys. 45, 143001(2012) and W. Zhou, D. Zhao, Y.-C. Shuai, H. Yang, S. Chuwongin, A.Chadha, J.-H. Seo, K. X. Wang, V. Liu, Z. Ma, and S. Fan, “Progress in2D photonic crystal Fano resonance photonics,” Prog. Quantum Electron.38, 1-74 (2014). After the single-crystalline Si layers weretransferred, a wet oxidation process was carried out at 1050° C. to growa 270 nm thick layer of SiO₂. This thermal oxidation consumed 124 nm ofthe single-crystalline Si layer. Thus, the final Si/SiO₂ layerthicknesses were exactly 110 nm and 270 nm. The transferred layers ofsingle-crystalline Si were bonded completely during the thermaloxidation process. Following the oxidation, another layer ofsingle-crystalline Si was transferred onto the top surface of the SiO₂layer. This cycle was repeated until the desired number of Si/SiO₂ pairswas formed. An optical microscope image of a 1 mm² sized Si/SiO₂ DBR ona quartz substrate is shown in FIG. 5. A cross-sectional SEM imageshowing two precisely defined and tightly bonded Si/SiO₂ layer pairs isshown in FIG. 4.

Reflection spectra were carefully taken using a custom build reflectionmeasurement system in connection with a Fourier transform infraredspectroscopy (FTIR) system at room temperature (˜23° C.). The simulatedand measured reflection spectra for a single-pair, a two-pair, and athree-pair Si/SiO₂ DBR on silicon substrates are shown in FIGS. 6, 7 and8, respectively. The measured spectra agree very well with the simulatedspectra and were blue shifted, closer to the simulated reflectionspectra, as the number of layer pairs was increased. On these substrateswith high refractive indexes, a high reflection, as high as 99.65% at awavelength of 1550 nm, was achieved by stacking three Si/SiO₂ layerpairs in a reflector stack having a total thickness of 1.14 μm. Thebandwidth over which the DBR maintained a reflectivity of 99.5% orhigher was about 270 nm (from 1400 to 1670 nm). For the Si/SiO₂ DBRs onthe quartz substrates, the measured and simulated reflection spectra areshown in FIGS. 9 and 10. A half pair was added for the DBRs on quartzsubstrates, because the initial DBR layer started with a 110 nm thicklayer of single-crystalline Si. The measured spectra agree well with thesimulated results and show less deviation from the simulated reflectionspectra. The total thickness of the 2.5-pair DBR on Si and the 3-pairDBR on quartz are 0.87 μm. Reflectivity as high as 99.65% at awavelength of 1550 nm was achieved by the 2.5-pair Si/SiO₂ reflectorstack, which had a total thickness is 0.87 μm. The bandwidth over whichthe DBR maintained a reflectivity of 99.5% or higher was about 400 nm(from 1300 to 1700 nm). Overall, both types of DBRs had reflectivitiesgreater than 99.4% with ˜1.2 μm at Full width at Half Maximum (FWHM).

As a comparative example, DBRs were made by forming multi-pairs ofpolycrystalline-Si/PECVD SiO₂ on quartz substrates. Thepolycrystalline-Si layers were deposited by a LPCVD under vacuum at 700°C. and the SiO₂ layers were deposited by PECVD at 350° C. Thereflectivity of these DBRs (shown in FIG. 11) reached >99% afterdepositing 5.5 layer pairs of poly-Si/SiO₂,which is comparable to thatof 2.5 pairs of single-crystalline Si/SiO₂ DBRs on a quartz substrate,but with more than twice the total thickness.

In summary, high quality and ultra-thin single-crystalline Si/SiO₂ DBRswere demonstrated with a thin layer silicon transfer-and-bonding processin combination with thermal oxidation of the silicon for SiO₂ growth.DBRs with different numbers of layer pairs were fabricated on both Siand quartz substrates, and their simulated and measured reflectance werecompared. A reflectivity of 99.65% in the wavelength range from 1350 nmto 1650 nm was measured from both a 2.5-layer pair DBR on a quartzsubstrate and a 3-layer pair DBR on a silicon substrate with totalthicknesses of 0.87 μm and 1.14 μm, respectively.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

1. A distributed Bragg reflector comprising: a reflector stackcomprising at least two adjacent layer pairs, each layer paircomprising: a layer of a single-crystalline Group IV semiconductor; andan adjacent layer of silicon dioxide; wherein the thickness of thereflector stack is no greater than 1 μm and the root mean squareroughness at the interfaces between the layers of single-crystallineGroup IV semiconductor and the layers of silicon dioxide is no greaterthan 0.5 nm.
 2. The reflector of claim 1, wherein the thickness of eachlayer pair is no greater than 450 nm and the layers of silicon dioxideare at least twice as thick as the layers of single-crystalline silicon.3. The reflector of claim 1, the reflector stack having from two and ahalf to three adjacent layer pairs.
 4. The reflector of claim 1, whereinthe single-crystalline Group IV semiconductor is silicon.
 5. Thereflector of claim 4, wherein the thickness of each layer pair is nogreater than 650 nm and the layers of silicon dioxide are at least twiceas thick as the layers of single-crystalline silicon.
 6. The reflectorof claim 5, wherein the layer of single-crystalline silicon has athickness of about 110 nm and the layer of silicon dioxide has athickness of about 270 nm and further wherein the reflector reflectslight having a wavelength of 1.55 μm.
 7. A distributed Bragg reflectorcomprising: a reflector stack comprising at least two adjacent layerpairs, each layer pair comprising: a layer of a single-crystalline GroupIV semiconductor; and an adjacent layer of silicon dioxide; wherein theroot mean square roughness at the interfaces between the layers ofsingle-crystalline Group IV semiconductor and the layers of silicondioxide is no greater than 0.5 nm; wherein the layer ofsingle-crystalline silicon has a thickness of about 175 nm and the layerof silicon dioxide has a thickness of about 435 nm and further whereinthe reflector reflects light having a wavelength of 0.98 μm.
 8. Thereflector of claim 1, wherein the single-crystalline Group IVsemiconductor is germanium.
 9. The reflector of claim 1, furthercomprising a substrate on which the stack of layer pairs is disposed,such that the lowermost layer of single-crystalline silicon in the stackis in contact with the substrate.
 10. The reflector of claim 9, whereinthe substrate is not silicon.
 11. The reflector of claim 1, thereflector stack characterized in that it provides a reflectivity of atleast 99.4% over the wavelength range from 1400 nm to 1670 nm on asilicon or quartz substrate.
 12. The reflector of claim 4, wherein thereflector has only three layer pairs and is characterized in that thatit provides a reflectivity of at least 99.5% at a wavelength within therange from 1400 to 1670 nm on a silicon substrate.
 13. The reflector ofclaim 4, the reflector stack comprising only two complete layer pairsand further comprising a layer of single-crystalline silicon on thesecond complete layer pair to provide a stack of 2.5 layer pairs,wherein the reflector is characterized in that that it provides areflectivity of at least 99.6% at a wavelength of 1550 nm on a quartzsubstrate.
 14. An array of reflectors comprising a plurality of thereflectors of claim 1 on a substrate, wherein different reflectors inthe array are configured to reflect light over different wavelengthranges.
 15. A vertical cavity surface emitting laser comprising: a lowerdistributed Bragg reflector comprising: a reflector stack comprising atleast two adjacent layer pairs, each layer pair comprising: a layer of asingle-crystalline Group IV semiconductor; and an adjacent layer ofsilicon dioxide; wherein the thickness of the reflector stack is nogreater than 1 μm and the root mean square at the interfaces between thelayers of single-crystalline Group IV semiconductor and the layers ofsilicon dioxide is no greater than 0.5 nm; an upper distributed Braggreflector comprising: a reflector stack comprising least two adjacentlayer pairs, each layer pair comprising: a layer of a single-crystallineGroup IV semiconductor; and an adjacent layer of silicon dioxide;wherein the thickness of the reflector stack is no greater than 1 μm andthe root mean square roughness at the interfaces between the layers ofsingle-crystalline Group IV semiconductor and the layers of silicondioxide is no greater than 0.5 nm; and a light-emitting active layerdisposed between the lower distributed Bragg reflector and the upperdistributed Bragg reflector.
 16. The laser of claim 15, wherein thelower and upper distributed Bragg reflectors provide a reflectivity ofat least 99.4% for light emitted by the active region in the wavelengthrange from 1300 nm to 1700 nm.
 17. The laser of claim 15, wherein theinnermost layer of single-crystalline Group IV semiconductor of theupper distributed Bragg reflectors is p-type doped to provide a p-typeohmic contact layer and the innermost layer of single-crystalline GroupIV semiconductor of the lower distributed Bragg reflector is n-typedoped to provide an n-type ohmic contact layer.
 18. An array of verticalcavity surface emitting lasers comprising a plurality of the lasers ofclaim 15 on a substrate, wherein different vertical cavity light surfaceemitting lasers in the array are configured to emit light at differentwavelengths when a voltage is applied across the vertical cavity surfaceemitting lasers.
 19. The array of claim 18, wherein somesingle-crystalline Group IV semiconductor is some of the vertical cavitysurface emitting lasers is silicon and the single-crystalline Group IVsemiconductor in some of the vertical cavity surface emitting lasers isgermanium.
 20. The array of claim 18, wherein some of the verticalcavity surface emitting lasers are configured to emit light at awavelength of 1.55 μm and some of the vertical cavity surface emittinglasers are configured to emit light at a wavelength of 0.98 μm.
 21. Thedistributed Bragg reflector of claim 1, wherein the single-crystallineGroup IV semiconductor is not a spin-on-glass type silicon oxidecomprising mixtures of non-stoichiometric silicon oxides.
 22. Thedistributed Bragg reflector of claim 7, wherein the single-crystallineGroup IV semiconductor is not a spin-on-glass type silicon oxidecomprising mixtures of non-stoichiometric silicon oxides.
 23. The laserof claim 15, wherein the single-crystalline Group IV semiconductors arenot spin-on-glass type silicon oxides comprising mixtures ofnon-stoichiometric silicon oxides.